Please do a proper fitting and do not just order off the internet. Which size you go with is dependent on 2 things... how it fits your daughter and her natural bouyancy. If she is a dense 29 pounds, then she will need the larger size. See if you have a scuba diving store in town that will allow you to try the vest with your daughter in it in the water.
Just going with the next size up could be dangerous if it doesn't fit properly. There is potential for your daughter to slip out of the vest, especially if there is a forceful event that lands her in the water.
For ocean you need at minimum a Type I PFD (personal flotation device). Type III (the most common vest style sold in stores) is not for open ocean use.
Please, please, please read:
http://www.uscg.mil/hq/cg5/cg5214/pfdselection.asp#recrea...